A summary of clinical data is presented on 34 men and 52 women patients with brain syndrome associated with delayed neuropsychiatric sequelae following acute carbon monoxide intoxication. Their ages ranged from 34 to 82 years, with peak incidence in the sixth and seventh decade. Possible etiological factors were age, duration of unconsciousness on acute intoxication, and previous physical illness. The onset was relatively sudden after the apparent clear period which ranged from 2 to 40 days (mean 22.5 days). The most frequent symptoms were apathy, dull facial expressions, dementia, such as amnesia and disorientation, hypokinesia, mutism, irritable distractibility, urinary and/or fecal incontinence, gait disturbance and abnormal neurological signs and reflexes. EEG was abnormal in 33 of the 57 cases (58%). Of 27 patients who were given a computed tomographic brain scan, 15 patients were abnormal. The prognosis was relatively good in the follow-up study of 56 patients. Only age was related to a better prognosis.